Quality of service analysis for hybrid-ARQ

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Abstract

Data intensive applications, requiring reliability and strict delay constraints,
have emerged recently and they necessitate a different approach to analyzing system
performance. In my work, I establish a framework that relates physical channel parameters
to the queueing performance for a single-user wireless system. I then seek to
assess the potential benefits of multirate techniques, such as hybrid-ARQ (Automatic
Repeat reQuest), in the context of delay-sensitive communications. Present methods
of analysis in an information theoretic paradigm define capacity assuming that
long codewords can be used to take advantage of the ergodic properties of the fading
wireless channel. This definition provides only a limited characterization of the channel
in the light of delay constraints. The assumption of independent and identically
distributed channel realizations tends to over-estimate the system performance by
not considering the inherent time correlation. A finite-state continuous time Markov
channel model that I formulate enables me to partition the instantaneous data-rate
received at the destination into a finite number of states, representing layers in a
hybrid-ARQ scheme. The correlation of channel has been incorporated through level
crossing rates as transition rates in the Markov model.
The large deviation principle governing the buffer overflow of the Markov model,
is very sensitive to channel memory, is tractable, and gives a good estimate of the
system performance. Metrics such as effective capacity and probability of buffer
overflow, that are obtained through large deviations have been related to the wireless
physical layer parameters through the model. Using the above metrics under QoS constraints, I establish the quantitative performance advantage of using hybrid-ARQ
over traditional systems. I conduct this inquiry by restricting attention to the case
where the expected transmit power is fixed at the transmitter. The results show that
hybrid-ARQ helps us in obtaining higher effective capacity, but it is very difficult to
support delay sensitive communication over wireless channel in the absence of channel
knowledge and dynamic power allocation strategies.